1. Field of Endeavor
The present invention relates to optics and more particularly to a system that mitigates the growth of surface damage in an optic.
2. State of Technology
U.S. Pat. No. 4,667,101 for predicting threshold and location of laser damage on optical surfaces by Wigbert Siekhaus, patented May 19, 1987 provides the following description, xe2x80x9cModern day applications of laser devices call for increasingly powerful and precise beams. Such applications require high resolution optical devices such as lenses, filters, and mirrors. The application of large intensities of laser energy to these devices frequently destroys them during operation. Often the level of intensity required for experimental applications (such as the Projects Nova and Novette at the Lawrence Livermore National Laboratory) is so high that pretesting of the optical device at the required intensities would be impractical. The level of effort required to prepare for and execute the desired experiments, however, is very high and so an effective means of pretesting such devices is desirable. Presently there are no commercially available devices capable of xe2x80x9cstress testingxe2x80x9d a particular optical device. U.S. Pat. No. 3,999,865, issued Dec. 28, 1976 to Milam, et al, teaches an instrument capable of analyzing the cause of damage to optical devices. It provides for subjecting the device to a damaging energy and intensity and then analyzing the damage from the standpoint of time and applied power in order to determine the one or more of several reasons for the laser induced damage. While Milam is helpful in improving system design or production techniques, it requires that damage actually occur and only indirectly identifies flaws through analysis of the parameters of the damaging event. The tested device clearly can no longer be used.xe2x80x9d
U.S. Pat. No. 5,796,523 for a laser damage control for optical assembly by John M. Hall, patented Aug. 18, 1998 provides the following description, xe2x80x9cProtection methods and apparatus for optical equipment have been attempted for providing protection from laser energy that could otherwise damage optical radiation detectors, including the human eye. The most common technique of providing protection involves optical filtering elements, which offer substantial protection but only over a limited, fixed spectral color range. Standard dielectric coatings are the most common form of filters, and flat plates with these xe2x80x9cnotchxe2x80x9d coatings can be easily inserted into or outside many common optical assemblies. As noted above, however, these filters are useful only over a limited range of wavelengths, and also have the added disadvantage of blocking even non-harmful radiation within the designed spectral region. Typical military magnifying optical assemblies such as telescopes, periscopes, and binoculars vary widely, and typically have magnifying powers ranging from 4xc3x97to 10xc3x97, with entrance aperture diameters going from 20 mm to 60 mm or more. As the magnifying power increases, the angular resolution increases, and thus the farther away a given target can be recognized. The larger apertures are required to gather sufficient light energy to allow good contrast for far-away targets. These magnifying optical systems are commonly designed for use with the human eye, but can also easily perform similar tasks when connected to standard television camera equipment. Given the harsh nature of military environments, these optical systems do not lend themselves easily to the use of attachments to perform laser protection functions. All magnifying optical assemblies of the kind found in telescopes, periscopes, and binoculars can be characterized as consisting of an objective lens set, followed by an eyepiece assembly, with either a real or virtual focal plane between, as well as a variety of intervening prism assemblies (almost always porro prisms) to keep the image orientation proper. The magnifying power is defined as the ratio of the objective focal length divided by the eyepiece focal length. Typical fields of view for these systems range from 2xc2x0to 10xc2x0, depending upon the magnification. In the prior art for all these systems, the focal planes between the objective and eyepiece sections, or between any intervening relay optics, is not well corrected for aberrations. This does not affect the overall system performance, because the aberrations of the objective can be compensated by those of the eyepiece. It is much more difficult to design both objective and eyepiece optics to each have diffraction limited focal planes, and therefore this feature is not normally embraced by the current art. Additionally, since the magnifying power is the ratio of the objective and eyepiece focal lengths, it is desirable to have a relatively short focal length eyepiece to minimize the objective focal length for a given magnification. This reduces the overall size of the system, but does not offer much room between the eyepiece assembly and the intermediate focal plane. Because of this, prior art designs do not usually allow elements other than thin transmissive reticle plates to occupy the space in or near the intermediate focal plane. The prior art in developing laser protective devices offers many techniques, including sacrificial mirrors, transmissive optical power limiters, liquid cells, etc. These devices are generally designed to operate passively within an optical system until indicent optical radiation is of sufficiently high energy to activate the protective mechanism. In order to set the activation threshold below the damage threshold of the detector (human eye, TV camera, etc.), it is desirable to place the power limiter in or near a well corrected, diffraction limited focal plane. Additionally, the optical system must be able to accommodate the volume of the power limiter device, and be able to provide proper image orientation should the device create an image translation.xe2x80x9d
U.S. Pat. No. 6,099,389 for fabrication of an optical component by Nichols et al, patented Aug. 8, 2000 provides the following description: xe2x80x9cA method for forming optical parts used in laser optical systems such as high energy lasers, high average power lasers, semiconductor capital equipment and medical devices. The optical parts will not damage during the operation of high power lasers in the ultra-violet light range. A blank is first ground using a fixed abrasive grinding method to remove the subsurface damage formed during the fabrication of the blank. The next step grinds and polishes the edges and forms bevels to reduce the amount of fused-glass contaminants in the subsequent steps. A loose abrasive grind removes the subsurface damage formed during the fixed abrasive or xe2x80x9cblanchardxe2x80x9d removal process. After repolishing the bevels and performing an optional fluoride etch, the surface of the blank is polished using a zirconia slurry. Any subsurface damage formed during the loose abrasive grind will be removed during this zirconia polish. A post polish etch may be performed to remove any redeposited contaminants. Another method uses a ceria polishing step to remove the subsurface damage formed during the loose abrasive grind. However, any residual ceria may interfere with the optical properties of the finished part. Therefore, the ceria and other contaminants are removed by performing either a zirconia polish after the ceria polish or a post ceria polish etch.xe2x80x9d
U.S. Pat. No. 5,472,748 for permanent laser conditioning of thin film optical materials by Wolfe et al, patented Dec. 5, 1995 provides the following description: xe2x80x9cThe performance of high peak power lasers, such as those used for fusion research and materials processing, is often limited by the damage threshold of optical components that comprise the laser chain. In particular, optical thin films generally have lower damage thresholds than bulk optical materials, and therefore thin films limit the output performance of these laser systems. Optical thin films are used as high reflectors, polarizers, beam splitters and anti-reflection coatings. The Nova project at Lawrence Livermore National Laboratory is designed to study the use of lasers to produce fusion by inertial confinement. The 1.06 xcexcm wavelength Nova laser output is limited, in part, by the damage threshold of large aperture (approximately 1 m diameter) dielectric thin films coated on flat substrates. Proposed future fusion lasers require optical coatings with laser induced damage thresholds that exceed a fluence of 35 J/cm2 in 10 ns pulses at the 1.06 xcexcm wavelength. Fluence is defined in the specification and claims for a pulsed laser of a specified wavelength and specified pulse length as the energy per unit area delivered by a single pulse. Prior to the invention, the highest damage thresholds were in the range from 10-20 J/cm2 in a 10 ns pulse at the 1.06 xcexcm wavelength. Therefore, a method of increasing the laser damage threshold of dielectric optical thin films (or coatings) is needed.xe2x80x9d
The present invention provides a system of mitigating the growth of laser-induced surface damage in an optic. A damage site in the optic is initiated. The position of the initiated damage site is identified. A mitigation process is performed that removes the cause of subsequent growth of the damage site. In one embodiment of the invention the system mitigates the growth of surface damage in an optic exposed to high-power laser having a wavelength of 1060 nm or less. Damage sites in the optic are initiated, located, and then treated to stop growth of the damage sites. Damage to the optic is minimally initiated. The step of initiating a damage site in the optic includes a scan of the optic using a laser to initiate defects. The exact position of the initiated site is identified. A mitigation process is performed that locally or globally removes the cause of subsequent growth of the damaged site. The mitigation process may be performed locally or globally.
In another embodiment of the invention, the mitigation process on a fused silica optic is performed by locally etching each damage site to render it smoother and remove any laser-energy absorbing defects in each damage site. The etching is accomplished using local application of an acid solution or by exposing the damage site to a small-aperture plasma jet containing fluorine atoms. In another embodiment of the invention, the mitigation process on a fused silica optic is performed by globally etching the entire surface of the optic to render all damage sites on the surface of the optic smoother and remove any laser-energy absorbing defects in all of the damage sites. The etching is accomplished by dipping the entire optic in an acid solution or by exposing the entire surface of the optic to a large-aperture plasma jet containing fluorine atoms. In another embodiment of the invention, the mitigation process on a fused silica optic is performed with a CO2 laser to locally soften the material within and in the immediate vicinity of each damage site to anneal out each damage site. In another embodiment of the invention, the mitigation process on a fused silica optic is performed by scanning the entire surface of the optic with a CO2 laser to anneal out all the damage sites. In another embodiment of the invention, the mitigation process on a fused silica, KDP or DKDP optic is performed with a laser having a pulse length less than 10 ns to locally ablate laser-energy absorbing defect in the damage site. In another embodiment of the invention, the mitigation process on a KDP or DKDP optic is performed by passivation of the damage site by local water etching. In another embodiment of the invention, the mitigation process on a KDP or DKDP optic is performed by removing the damage site with a mechanical grinding tool.
Features of specific embodiments of the invention are to reduce of growth of catastrophic damage on the surface of fused silica, potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) optics, such that the optics can survive prolonged exposure to high-power laser beams having a wavelength of about 1060 nm or less. Another feature is to substantially improve the lifetime of optical components made of fused silica, KDP or DKDP, such that these optical components can survive prolonged exposure to high-power laser irradiation at wavelengths of about 1060 or less. Other features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating specific embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description and by practice of the invention.